1. Field of the Invention
The invention relates to the separation of gases. More particularly, it relates to the enhancing of the recovery of high purity product in such gas separation operations.
2. Description of the Prior Art
Permeable membranes capable of selectively permeating one component of a gas mixture have been considered as a convenient, potentially highly desirable means for achieving gas separations. The use of such membranes depends, to a considerable extent, on the differential pressures maintained on opposite sides thereof, with the passage of the more permeable component through the membrane being enhanced as the pressure differential across the membrane is increased. Such pressure differential is limited, however, by practical operating considerations, such as the strength of the membrane itself, compression costs applicable to such separations, and the like. Because of such factors, the purity and product recovery associated with the use of a permeable membrane tends to be limited to the upgrading of relatively high quality gas streams.
For gas separation operations in which a single stage membrane system is not capable of achieving the normal product purity levels desired in the art, two-stage permeable membrane systems have been proposed, wherein the concentration of the product gas is improved by passing the permeate first stage membrane to an additional membrane stage to improve the purity of the desired product gas. Thus, the Null patent, U.S. Pat. No. 4,264,338, illustrates a two-stage membrane system, with an inter-stage compressor, but indicates that even such a system is not capable of providing the normal purity product commonly desired in the art. It was also pointed out by Null that the use of additional membrane stages, in a so-called gas separation cascade, would severely increase operating costs and compressor, membrane and related equipment costs. In many instances, therefore, the costs needed to achieve a normal purity product gas were said by Null to increase beyond the point of economic feasibility. As a result, despite the recognized convenience and other advantages pertaining to the membrane approach, permeable membranes were not employed for many practical commercial gas separation operations.
In an effort to overcome such a restriction concerning the practical use of membranes, Null proposed a three-stage membrane system in which the non-permeate gas from the second of two membranes employed in series, with inter-stage compressor, is passed at elevated pressure to a third stage, or recycle stage, membrane. The permeate recovered from this stage is blended with the first stage permeate prior to the recompression thereof by the in-line, inter-stage compressor, and the passage of such compressed gas to the second stage membrane. In this approach, the non-permeate gas from the third stage membrane, available at elevated pressure, is blended with additional quantities of the feed gas mixture for recycle to the first stage membrane. The non-permeate gas from said first stage membranes is the only waste gas removed from the system, while the desired normal purity product gas comprises the permeate gas recovered at reduced pressure from the second stage membrane.
The Null approach employing a recycle membrane stage can be used in some embodiments to produce a normal purity gas, i.e., about 95-97% purity in the case of hydrogen or helium purification, not previously obtainable in practical commercial operations by the use of permeable membranes for gas separation. It will be appreciated that the recycle membrane is of value in this regard only when the permeation pressures and other factors are such as to enable the recycle membrane to more effectively separate the desired permeate portion of the gas mixture recycled from the second stage membrane than is accomplished in said first stage itself. A greater driving force across the recycle membrane is required for this purpose. Thus, a balancing of membrane and compression costs is necessarily involved in this approach, as in other approaches to the use of permeable membranes for gas separation.
While membranes can be conveniently employed for the upgrading of low quality gas streams, and can be used effectively to produce the normal purity product referred to above by use of the Null multi-stage approach, membranes have not been effectively employed to separate a high purity product at desirably high recovery levels. A high purity product, for purposes hereof and as known in the art, typically comprises a product gas having a purity of from about 98% to about 99.9.sup.+ mol % in the case of hydrogen or helium purification. Thus, the three stage system of Null is not adapted for the production of such high purity gas, and, as noted above, the use of additional membrane stages to achieve high purity levels is not feasible because of the prohibitive costs associated with such additional staging. There remains a desire in the art, therefore, for a gas separation process utilizing the convenience of permeable membranes while producing high purity product gas at desirable recovery levels.
It is an object of the invention, therefore, to provide an improved gas separation process utilizing the convenience of permeable membranes.
It is another object of the invention to provide a gas separation process utilizing permeable membranes, but also capable of recovering high purity product gas at desirable recovery levels.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.